Cervical disc replacement

ABSTRACT

An apparatus for replacing at least a portion of an intervertebral disc in a spinal column includes: a first member having a first vertebral contact surface for engagement with an endplate of a first vertebral bone in the spinal column and a first articulating surface having a single saddle surface; and a second member having a second vertebral contact surface for engagement with an endplate of a second vertebral bone in the spinal column and a second articulating surface having a single saddle surface wherein: an intervertebral disc space is defined substantially between the first and second endplates of the first and second vertebral bones, and the first and second members are operable to articulate relative to one another, when disposed in the intervertebral disc space, about at least one of: (i) a first center of rotation for at least one of flexion and extension that is located above the first and second articulating surfaces outside the intervertebral disc space, and (ii) a second center of rotation for lateral bending that is located below the first and second articulating surface.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. patent application Ser. No.10/382,702, filed Mar. 6, 2003 now U.S. Pat. No. 6,908,484, entitledCERVICAL DISC REPLACEMENT, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention is directed to a cervical joint replacementimplant and more particularly to a cervical intervertebral discprosthesis having opposing constant radii saddle shaped articulatingsurfaces.

The structure of the intervertebral disc disposed between the cervicalbones in the human spine comprises a peripheral fibrous shroud (theannulus) which circumscribes a spheroid of flexibly deformable material(the nucleus). The nucleus comprises a hydrophilic, elastomericcartilaginous substance that cushions and supports the separationbetween the bones while also permitting articulation of the twovertebral bones relative to one another to the extent such articulationis allowed by the other soft tissue and bony structures surrounding thedisc. The additional bony structures that define pathways of motion invarious modes include the posterior joints (the facets) and the lateralintervertebral joints (the unco-vertebral joints). Soft tissuecomponents, such as ligaments and tendons, constrain the overallsegmental motion as well.

Traumatic, genetic, and long term wearing phenomena contribute to thedegeneration of the nucleus in the human spine. This degeneration ofthis critical disc material, from the hydrated, elastomeric materialthat supports the separation and flexibility of the vertebral bones, toa flattened and inflexible state, has profound effects on the mobility(instability and limited ranges of appropriate motion) of the segment,and can cause significant pain to the individual suffering from thecondition. Although the specific causes of pain in patients sufferingfrom degenerative disc disease of the cervical spine have not beendefinitively established, it has been recognized that pain may be theresult of neurological implications (nerve fibers being compressed)and/or the subsequent degeneration of the surrounding tissues (thearthritic degeneration of the facet joints) as a result of their beingoverloaded.

Traditionally, the treatment of choice for physicians caring forpatients who suffer from significant degeneration of the cervicalintervertebral disc is to remove some, or all, of the damaged disc. Ininstances in which a sufficient portion of the intervertebral discmaterial is removed, or in which much of the necessary spacing betweenthe vertebrae has been lost (significant subsidence), restoration of theintervertebral separation is required.

Unfortunately, until the advent of spine arthroplasty devices, the onlymethods known to surgeons to maintain the necessary disc heightnecessitated the immobilization of the segment. Immobilization isgenerally achieved by attaching metal plates to the anterior orposterior elements of the cervical spine, and the insertion of someosteoconductive material (autograft, allograft, or other porousmaterial) between the adjacent vertebrae of the segment. Thisimmobilization and insertion of osteoconductive material has beenutilized in pursuit of a fusion of the bones, which is a procedurecarried out on tens of thousands of pain suffering patients per year.

This sacrifice of mobility at the immobilized, or fused, segment,however, is not without consequences. It was traditionally held that thepatient's surrounding joint segments would accommodate any additionalarticulation demanded of them during normal motion by virtue of thefused segment's immobility. While this is true over the short-term(provided only one, or at most two, segments have been fused), theeffects of this increased range of articulation demanded of theseadjacent segments has recently become a concern. Specifically, anincrease in the frequency of returning patients who suffer fromdegeneration at adjacent levels has been reported.

Whether this increase in adjacent level deterioration is trulyassociated with rigid fusion, or if it is simply a matter of theindividual patient's predisposition to degeneration is unknown. Eitherway, however, it is clear that a progressive fusion of a long sequenceof vertebrae is undesirable from the perspective of the patient'squality of life as well as from the perspective of pushing a patient toundergo multiple operative procedures.

While spine arthroplasty has been developing in theory over the pastseveral decades, and has even seen a number of early attempts in thelumbar spine show promising results, it is only recently thatarthoplasty of the spine has become a truly realizable promise. Thefield of spine arthroplasty has several classes of devices. The mostpopular among these are: (a) the nucleus replacements, which arecharacterized by a flexible container filled with an elastomericmaterial that can mimic the healthy nucleus; and (b) the total discreplacements, which are designed with rigid endplates which house amechanical articulating structure that attempts to mimic and promote thehealthy segmental motion.

Among these solutions, the total disc replacements have begun to beregarded as the most probable long-term treatments for patients havingmoderate to severe lumbar disc degeneration. In the cervical spine, itis likely that these mechanical solutions will also become the treatmentof choice. At present, there are two devices being tested clinically inhumans for the indication of cervical disc degeneration. The first ofthese is the Bryan disc, disclosed in part in U.S. Pat. No. 6,001,130.The Bryan disc is comprised of a resilient nucleus body disposed inbetween concaval-covex upper and lower elements that retain the nucleusbetween adjacent vertebral bodies in the spine. The concaval-convexelements are L-shaped supports that have anterior wings that acceptbones screws for securing to the adjacent vertebral bodies.

The second of these devices being clinically tested is the Bristol disc,disclosed substantially in U.S. Pat. No. 6,113,637. The Bristol disc iscomprised of two L-shaped elements, with corresponding ones of the legsof each element being interposed between the vertebrae and in oppositionto one another. The other of the two legs are disposed outside of theintervertebral space and include screw holes through which the elementsmay be secured to the corresponding vertebra; the superior element beingsecured to the upper vertebral body and the inferior element beingattached to the lower vertebral body. The opposing portions of each ofthe elements comprise the articulating surfaces that include anelliptical channel formed in the lower element and a convexhemispherical structure disposed in the channel.

As is evident from the above descriptions, the centers of rotation forboth of these devices, which are being clinically tested in humansubjects, is disposed at some point in the disc space. More particularlywith respect to the Bryan disc, the center of rotation is maintained ata central portion of the nucleus, and hence in the center of the discspace. The Bristol disc, as a function of its elongated channel (itselongated axis being oriented along the anterior to posteriordirection), has a moving center of rotation which is, at all timesmaintained within the disc space at the rotational center of thehemispherical ball (near the top of the upper element).

Other aspects, features and advantages of the present invention notalready evident will become evident from the description herein taken inconjunction with the accompanying drawings.

SUMMARY OF THE INVENTION

The present invention provides an articulating joint implant thatincludes a pair of opposing upper and lower elements having nestedarticulation surfaces providing a center of rotation of the implantabove the adjacent vertebral body endplate surfaces in one mode ofmotion (e.g., lateral bending) and a center of rotation of the implantbelow those surfaces in another mode of motion (e.g.,flexion/extension), and that further permit axial rotation of theopposing elements relative to one another (for example, about an axissuch as, for example, a longitudinal axis, for example, of the spinalcolumn)) through a range of angles without causing them to move indirections that are directed away from one another (for example, inopposing directions along the axis of axial rotation) within that range.In preferred embodiments, the articulation surfaces further cause suchopposite (or otherwise directed away from one another) movement of theopposing elements beyond that range.

More particularly, the present invention contemplates that with regardto the cervical anatomy, a device that maintains a center of rotation,moving or otherwise, within the disc space is inappropriate and fails toproperly support healthy motion. Specifically, the cervical jointcomprises five separate articulating elements: the facet joints in theposterior of the segment; the lateral unco-vertebral joints; and thenucleus in the intervertebral space. It is contemplated by the presentinvention that a track defined by the cervical facets falls along theplanes between the inferior surface of the upper facets and the superiorsurface of the lower facets, and that this plane extends upwardly andforward, forcing the overall joint to pivot around a center of rotationthat resides in the lower vertebral bone in flexion/extensionarticulation.

Conversely, it is contemplated by the present invention that in lateralbending the unco-vertebral joints influence the track of motion.Specifically, the unco-vertebral joints are formed at the lateral edgesof the intervertebral space and are defined by a pair of upwardlyextending surfaces of the inferior vertebral endplate and thecorresponding surfaces of the superior bony endplate. It is contemplatedby the present invention that this U-shaped configuration guides thesegment into a rotation about a center within the superior vertebralbone during lateral bending.

Finally, it is contemplated by the present invention that during axialrotation of the adjacent vertebral bones of the cervical segmentrelative to one another about the longitudinal axis of the spinalcolumn, the opposing bones do not simply axially rotate relative to oneanother for more than a few degrees, but rather follow the coupledinfluences of the unco-vertebral joints and the nucleus, and that thiscoupled motion vertically separates the opposing bones of the facetjoints as the rotation continues, thus freeing the bones to rotatefarther that would otherwise be permitted if the facets locked together(as is often seen as a symptom of degenerative cervical disease).

Both the Bryan and Bristol discs described above do provide distractionand maintenance of intervertebral disc height, and thereby provideimmediate and short-term relief from pain. However, it should beunderstood, in light of the above described anatomy as contemplated bythe present invention, that neither provides for proper anatomicalmotion because their respective centers of rotation are located withinthe disc space. Thus, neither will afford significant motionpreservation, and patients with these devices implanted in their neckswill find no significant mobility at the implanted segment. This maylead to spontaneous fusions, long term facet deterioration, and/oraccelerated adjacent level degeneration.

The constraints placed on the prosthesis by the above-described anatomyare considerable. To provide an implant having a pair of articulationsurfaces that provide a center of rotation of the implant above thesurfaces in one mode of motion (lateral bending) and a center ofrotation of the implant below the surfaces in another mode of motion(flexion/extension), that further permit the surfaces to axially rotaterelative to one another about a longitudinal axis of the spinal columnthrough a range of angles without causing movement of the surfaces inopposing directions along the longitudinal axis of the spinal column,and that further cause such movement (and accordingly a verticalseparation of the facet joints) beyond that range is a difficultengineering task. The present invention contemplates that a saddle jointprovides a geometric approach to the task.

The solution to this problem, however, is not open to just any saddlejoint configuration. U.S. Pat. Nos. 5,405,400 and 5,645,605 describesaddle joints utilized for prosthetic thumb joints. More particularly,U.S. Pat. No. 5,405,400 (“Linscheid”) discloses an artificial thumbjoint comprising a pair of surfaces that are nesting hyperbolicparaboloids. A hyperbolic paraboloid is a surface defined by a firstspecific geometric form (the hyperbola) that is swept along a secondgeometric form (the parabola) that is perpendicular to the first form,and which first and second forms are opposite in the direction of theirconvexities. A common feature of both hyperbolae and parabolas is thatneither has a constant radius of curvature along its extent. Constantradii of curvature are necessary for a pair of surfaces to smoothly flowover one another. Accordingly, the nesting hyperbolic paraboloids setforth in the reference are, therefore, not capable of any smootharticulation. Any attempted articulation causes the two surfaces toimmediately move in opposing directions. Stated more simply, nestinghyperbolic paraboloids have continuously changing centers of rotation(by the nature of the geometric forms selected). The present inventioncontemplates that the cervical joint anatomy enables smooth articulationin two modes of motion (lateral bending and flexion/extension), and alsosmooth relative axial rotation about the longitudinal axis of the spinalcolumn through a small range of angles. It is understood by the presentinvention that the vertical separation motion of the natural cervicaljoint does not occur immediately, but rather occurs only outside thatsmall angular range of relative axial rotation. Thus, the presentinvention contemplates that the nesting hyperbolic paraboloids disclosedby Linscheid are inappropriate for use in the cervical joint.

U.S. Pat. No. 5,645,605 (“Klawitter”) discloses an alternate form of asaddle surface, again for use in an artificial thumb joint, thatcomprises a pair of nesting toroidal surfaces. Toroidal surfaces aredefined by an arc of one circle being swept along an arc of another,again having opposing convexities. As circles have constant radii ofcurvatures, it is possible with these surfaces to have smooth motion intwo perpendicular planes. More particularly, if the corresponding radiiof curvature are approximately equivalent, the two surfaces may nest andarticulate in flexion/extension and lateral bending smoothly, withoutcausing the surfaces to move in opposing directions upon an attemptedarticulation. However, Klawitter teaches that these toroidal surfacesshould have the same radii of curvature, inasmuch as it is a necessitythat axial rotational motion of the joint be inhibited, or if it ispermitted to occur, it should cause an immediate axial distraction ofthe joint. As explained above with regard to the saddle joint inLinscheid, this elimination of the capacity of the surfaces to axiallyrotate relative to one another for even a small range of angles preventssuch a design from being effectively used in a cervical discapplication.

In order for two nesting toroidal saddles to rotate within the sameplane, each of the concave radii of the surfaces must be greater thanthe radius of its nested convex surface. An artificial cervical jointthat provides a center of rotation in the vertebral bone below duringflexion/extension and in the vertebral bone above during lateral bendingand has the capacity to axially rotate within a small range of anglesprior to causing oppositely directed movement of the surfaces requiresnesting surfaces with such a configuration.

The present invention, therefore, provides an articulating joint implantfor use in the cervical spine, including a first (e.g., upper) elementand a second (e.g., lower) element, each having an outwardly facingvertebral body contact surface, and each having an inwardly facingarticulation surface. The elements are disposed with the articulationsurfaces nested against one another, and the vertebral body contactsurfaces facing away from one another. When the implant is disposed inan intervertebral disc space in a cervical spine, in this configurationand with the vertebral body contact surfaces engaged with respectiveadjacent vertebral body endplates, the implant enables the adjacentvertebral bones to move relative to one another in accordance withproper anatomical motion.

Preferably, each of the elements has at least one long-term fixationstructure (e.g., a flange) having at least one feature (e.g., a throughhole) for securing the element to an adjacent vertebral body. Forexample, the upper element has an anterior flange that extends upwardlyand has two through holes, each of which accepts a bone screw. And, forexample, the lower element has an anterior flange that extendsdownwardly and has one through hole that accepts a bone screw. Furtherpreferably, each of the elements has at least one short-term fixationstructure (e.g., spikes on the outwardly directed vertebral body contactsurface) for securing the element to an adjacent vertebral bodyendplate.

Further preferably, each of the outwardly directed vertebral bodycontact surfaces is shaped to conform to the endplate of an adjacentvertebral body against which it is to be positioned. For example,vertebral body contact surface of the upper element is curvate (to matchthe anatomy of the superior vertebral body endplate) and the vertebralbody contact surface of the lower element is flat (to match the anatomyof the inferior vertebral body endplate). Further preferably, eachvertebral body contact surface has an osteoinductive or osteoconductivefeature, such as, for example, porous or rough areas.

The longitudinally inwardly directed articulation surface of the upperelement forms a constant radii saddle-shaped articulation surface. Moreparticularly, the saddle surface is defined by a concave arc that isswept perpendicular to and along a convex arc. The articulation surfacehas a cross-section in one plane that forms a concave arc, and across-section in another plane (perpendicular to that plane) that formsa convex arc. The concave arc has a respective constant radius ofcurvature about an axis perpendicular to the one plane. The convex archas a respective constant radius of curvature about an axisperpendicular to the other plane.

In a preferred embodiment, the concave arc has a constant radius ofcurvature A about an axis perpendicular to the anterior-posterior plane,and the convex arc has a constant radius of curvature B about an axisperpendicular to the lateral plane. Preferably, radius A is less thanradius B.

The longitudinally inwardly directed articulation surface of the lowerelement also forms a constant radii saddle-shaped articulation surface.More particularly, the saddle surface is defined by a convex arc that isswept perpendicular to and along a concave arc. The articulation surfacehas a cross-section in one plane that forms a convex arc, and across-section in another plane (perpendicular to that plane) that formsa concave arc. The convex arc has a respective constant radius ofcurvature about an axis perpendicular to the one plane. The concave archas a respective constant radius of curvature about an axisperpendicular to the other plane.

In a preferred embodiment, the convex arc has a constant radius ofcurvature C about an axis perpendicular to the anterior-posterior plane,and the concave arc has a constant radius of curvature D about an axisperpendicular to the lateral plane. Preferably, radius C is less thanradius D.

The constant radii saddle shaped articulation surfaces are configuredand sized to be nestable against one another and articulatable againstone another, to enable adjacent vertebral bones (against which the upperand lower elements are respectively disposed in the intervertebralspace) to articulate in flexion, extension, and lateral bending. Moreparticularly, the artificial disc implant of the present invention isassembled by disposing the upper and lower elements such that thevertebral body contact surfaces are directed away from one another, andthe articulation surfaces are nested against one another such that theconcave arcs accommodates the convex arcs.

Accordingly, movement of the adjacent vertebral bones relative to oneanother is permitted by the movement of the upper and lower elementsrelative to one another. In flexion and extension, the concave arcs ofthe upper element ride on the convex arcs of the lower element about acenter of rotation below the articulation surfaces. In lateral bending,the concave arcs of the lower element ride on the convex arcs of theupper element about a center of rotation above the articulationsurfaces. During these articulations, the elements are maintained atconstant relative distraction positions, i.e, the elements do not movein directions that are directed away from one another (for example, donot move in opposing axial directions from one another (e.g., along alongitudinal axis of the spine)). Accordingly, the present inventionprovides a pair of articulation surfaces that have a center of rotationabove the surfaces in one mode of motion (lateral bending), and belowthe surfaces in another (flexion/extension), consistent in these regardswith a natural cervical intervertebral joint. Preferably, thearticulation surfaces are sized and configured so that the respectiveranges of angles through which flexion/extension and lateral bending canbe experienced are equal to or greater than the respective normalphysiologic ranges for such movements in the cervical spine.

It is preferable that, in addition to the flexion, extension, andlateral bending motions described above, the adjacent vertebral bones bepermitted by the artificial disc implant to axially rotate relative toone another (e.g., about the longitudinal axis of the spinal column),through a small range of angles, without moving in opposite (orotherwise directed away from one another) directions (e.g., along thelongitudinal axis) within that range, and then to engage in suchopposite (or otherwise directed away from one another) movement oncethat range is exceeded. Preferably, the articulation surfaces 204, 304are accordingly configured and sized to permit such movements. In apreferred configuration, the constant radius of curvature A is largerthan the constant radius of curvature C, and the constant radius ofcurvature D is larger than the constant radius of curvature B. Becauseof the space, afforded by the differing radii, at the edges of thearticulation surfaces, the upper and lower elements are able to axiallyrotate relative to one another about the longitudinal axis of the spinalcolumn through a range of angles without causing the vertebral bodycontact surfaces to move away from one another along the longitudinalaxis. Once the axial rotation exceeds that range, the articulationsurfaces interfere with one another as the concave arcs move towardpositions in which they would be parallel to one another, and thedistance between the vertebral body contact surfaces increases withcontinued axial rotation as the concave arcs ride up against theiroppositely directed slopes. Thus, the articulation surfaces areconfigurable according to the present invention to permit normalphysiologic axial rotational motion of the adjacent vertebral bonesabout the longitudinal axis through a range of angles without abnormalimmediate axially opposite (or otherwise directed away from one another)movement, and to permit such axially opposite (or otherwise directedaway from one another) movement when under normal physiologic conditionsit should occur, that is, outside that range of angles.

In preferred embodiments where the constant radius of curvature A islarger than the constant radius of curvature C, and the constant radiusof curvature D is larger than the constant radius of curvature B, thearticulation surfaces maintain point-to-point contact over a range ofnormal physiologic articulating movement between the adjacent vertebralbones. That is, through flexion, extension, lateral bending, and axialrotation, the articulation surfaces are in point-to-point contact withone another.

Preferably, the surface area dimensions of the articulation surfaces areselected in view of the selected radii of curvature to prevent the edgesof the saddle surfaces (particularly the edges of the concave arcs) fromhitting any surrounding anatomic structures, or other portions of theopposing upper or lower element, before the limit of the normalphysiologic range of an attempted articulation is reached.

In accordance with one or more aspects of the present invention, anapparatus for replacing at least a portion of an intervertebral disc ina spinal column includes: a first member having a first vertebralcontact surface for engagement with an endplate of a first vertebralbone in the spinal column, and having a first articulation surface; anda second member having a second vertebral contact surface for engagementwith an endplate of a second vertebral bone in the spinal column, andhaving a second articulation surface, wherein: an intervertebral discspace is defined substantially between the first and second endplates ofthe first and second vertebral bones, and at least one of the first andsecond articulation surfaces is a saddle shaped surface, and thearticulation surfaces are sized and shaped to engage one another whenthe first and second members are disposed in the intervertebral discspace to enable the first and second vertebral bones to articulate in atleast one of flexion, extension, and lateral bending.

The novel features of the present invention, as well as the inventionitself, both as to its structure and its operation will be understoodfrom the accompanying drawings, taken in conjunction with theaccompanying description, in which similar reference characters refer tosimilar parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1–5 show an artificial disc implant of the present invention inperspective, anterior, lateral, lateral cutaway, and posterior cutawayviews, respectively.

FIGS. 6–12 show an upper element of the artificial disc implant of FIGS.1–5 in perspective, bottom (looking longitudinally up), lateral,anterior, lateral cutaway, top (looking longitudinally down), andposterior cutaway views, respectively.

FIGS. 13–19 show a lower element of the artificial disc implant of FIGS.1–5 in perspective, top (looking longitudinally down), lateral,anterior, lateral cutaway, bottom (looking longitudinally up), andposterior cutaway views, respectively.

FIG. 20 shows a lateral cross-section view of the artificial discimplant of FIGS. 1–5 in extension.

FIG. 21 shows a posterior cross-section view of the artificial discimplant of FIGS. 1–5 in lateral bending.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein, beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Referring now to FIGS. 1–5, an artificial disc implant 100 of thepresent invention is shown in perspective, anterior, lateral, lateralcutaway, and posterior cutaway views, respectively. The implant 100includes a first (e.g., upper) element 200 and a second (e.g., lower)element 300, each having an outwardly facing vertebral body contactsurface 202, 302, and each having an inwardly facing articulationsurface 204, 304. The elements 200, 300 are disposed as shown with thearticulation surfaces 204, 304 nested against one another, and thevertebral body contact surfaces 202, 302 facing away from one another.When the implant 100 is disposed in an intervertebral disc space in acervical spine, in this configuration and with the vertebral bodycontact surfaces 202, 302 engaged with respective adjacent vertebralbody endplates (not shown), the implant 100 enables the adjacentvertebral bones to move relative to one another in accordance withproper anatomical motion, as further described below.

Preferably, at least one (and more preferably both) of the elements 200,300 has at least one long-term fixation structure (e.g., flange 206,306) having at least one feature (e.g., through hole 208 a, 208 b, 308)for securing the element to an adjacent vertebral body. For example theupper element 200 has an anterior flange 206 that extends upwardly andhas two through holes 208 a, 208 b, each of which accepts a bone screw(not shown). And, for example, the lower element 300 has an anteriorflange 306 that extends downwardly and has one through hole 308 thataccepts a bone screw (not shown). Once the elements 200, 300 aredisposed in the intervertebral space with the vertebral body contactsurfaces 202, 302 engaged with respective adjacent vertebral bodyendplates (not shown), securing of bone screws through the holes 208 a,208 b, 308 and into the anterior surfaces of the adjacent vertebralbones helps prevent the elements from becoming dislodged from, ordisplaced in, the intervertebral space. Preferably, the bore axes of thethrough holes 208 a, 208 b, 308 are angled toward the adjacent vertebralbody as shown.

Further preferably, at least one (and more preferably both) of theelements 200, 300 has at least one short-term fixation structure (e.g.,spike 210 a, 210 b, 310 a, 310 b) for securing the element to anadjacent vertebral body (and more preferably to an adjacent vertebralbody endplate). For example, each of the elements 200, 300 has arespective pair of outwardly directed spikes 210 a, 210 b, 310 a, 310 b.Once the elements 200, 300 are disposed in the intervertebral space withthe vertebral body contact surfaces 202, 302 engaged with respectiveadjacent vertebral body endplates (not shown), the spikes 210 a, 210 b,310 a, 310 b dig into the adjacent vertebral body endplates under thecompression along the longitudinal axis of the spinal column, and thushelp prevent the elements from becoming dislodged from, or displaced in,the intervertebral space. Preferably, each of the spikes 210 a, 210 b,310 a, 310 b is sloped toward the vertebral body contact surface 202,302 and toward the posterior direction on its posterior side as shown,to facilitate ease of insertion of the implant 100 into theintervertebral space, and is either perpendicular to the vertebral bodycontact surface 202, 302 on its anterior side (as shown) or slopedtoward the vertebral body contact surface 202, 302 and toward theposterior direction on its anterior side (not shown), to help preventthe elements 200, 300 from anteriorly (or otherwise) slipping out of theintervertebral space.

More particularly, and referring now to FIGS. 6–12, the upper element200 of the artificial disc implant 100 shown in FIGS. 1–5 is shown inperspective, bottom (looking longitudinally up), lateral, anterior,lateral cutaway, top (looking longitudinally down), and posteriorcutaway views, respectively Further particularly, and referring now toFIGS. 13–19, the lower element 300 of the artificial disc implant 100shown in FIGS. 1–5 is shown in perspective, top (looking longitudinallydown), lateral, anterior, lateral cutaway, bottom (lookinglongitudinally up), and posterior cutaway views, respectively.

As introduced above, each of the elements 200, 300 has a longitudinallyoutwardly directed vertebral body contact surface 202, 302. Preferably,each surface 202, 302 is shaped to conform to an endplate of an adjacentvertebral body (not shown) against which it is to be positioned. Forexample, inasmuch as a review of the relevant anatomy indicates thatlower endplates of vertebral bones in the cervical spine each have acentral concavity, it is preferable that the surface 202 is curvate (andmore preferably, domed as shown, or semi-cylindrical), to conform to thecentral concavity. And, for example, inasmuch as a review of therelevant anatomy indicates that upper endplates of vertebral bones inthe cervical spine are generally flat, it is preferable that the surface302 is flat, as shown. It should be understood that the surfaces 202,302 can be formed or can be dynamically formable to have these or othershapes that closely conform to the adjacent vertebral body endplate,without departing from the scope of the present invention.

Each vertebral body contact surface 202, 302 further preferably has anosteoinductive or osteoconductive feature. For example, each surface202, 302 is preferably porous and/or roughened. This can be accomplishedby any manner known in the art, including, for example, grit blasting,porous coating, etching, burning, electrical discharge machining, andsintered beading. While both surfaces 202, 302 are preferably providedwith such a feature, it should be understood that only one could havesuch a feature without departing from the scope of the presentinvention. Further, it should be understood that it is not necessary forthe entire surface to be so featured, but rather only a portion, someportions, or all of the surface can be so featured, or have a variety ofsuch features, without departing from the scope of the presentinvention.

Each vertebral body contact surface 202, 302 further preferably has thelong-term fixation and short-term fixation structures described aboveand denoted by corresponding reference numbers on these FIGS. 6–19.

As introduced above, the upper element 200 has a longitudinally inwardlydirected articulation surface 204. Preferably, as shown, thearticulation surface 204 includes a saddle surface that is defined by aconcave arc (denoted by reference numeral 212 on FIG. 10) that is sweptperpendicular to and along a convex arc (denoted by reference numeral214 on FIG. 12). As best seen in FIGS. 4, 5, 10, and 12, thearticulation surface 204 has a cross-section in one plane that forms aconcave arc 212, and a cross-section in another plane (perpendicular tothat plane) that forms a convex arc 214. The concave arc 212 has arespective substantially constant radius of curvature about an axisperpendicular to the one plane. The convex arc 214 has a respectivesubstantially constant radius of curvature about an axis perpendicularto the other plane. Therefore, the articulation surface 204 forms asubstantially constant radii saddle-shaped articulation surface.

In this preferred embodiment, as indicated in FIG. 10, the concave arc212 has a substantially constant radius of curvature A about an axisperpendicular to the anterior-posterior plane. And, in this preferredembodiment, as indicated in FIG. 12, the convex arc 214 has asubstantially constant radius of curvature B about an axis perpendicularto the lateral plane. Preferably, radius A is less than radius B, andmost preferably, radius A is 0.329 and radius B is 0.340. It should benoted, however, that the present invention is not limited to anyparticular dimension set, and further than in some embodiments of thepresent invention, radius A is equal to or greater than radius B.

Also as introduced above, the lower element 300 has a longitudinallyinwardly directed articulation surface 304. Preferably, as shown, thearticulation surface 304 includes a saddle surface that is defined by aconvex arc (denoted by reference numeral 312 on FIG. 17) that is sweptperpendicular to and along a concave arc (denoted by reference numeral314 on FIG. 19). As best seen in FIGS. 4, 5, 17, and 19, thearticulation surface 304 has a cross-section in one plane that forms aconvex arc 312, and a cross-section in another plane (perpendicular tothat plane) that forms a concave arc 314. The convex arc 312 has arespective substantially constant radius of curvature about an axisperpendicular to the one plane. The concave arc 314 has a respectivesubstantially constant radius of curvature about an axis perpendicularto the other plane. Therefore, the articulation surface 304 also forms asubstantially constant radii saddle-shaped articulation surface.

In this preferred embodiment, as indicated in FIG. 17, the convex arc312 has a substantially constant radius of curvature C about an axisperpendicular to the anterior-posterior plane. And, in this preferredembodiment, as indicated in FIG. 19, the concave arc 314 has asubstantially constant radius of curvature D about an axis perpendicularto the lateral plane. Preferably, radius C is less than radius D, andmost preferably, radius C is 0.280 inches and radius D is 0.401 inches.It should be noted, however, that the present invention is not limitedto any particular dimension set, and further than in some embodiments ofthe present invention, radius C is equal to or greater than radius D.Further in some embodiments, radii A, B, C, and D are of equaldimension.

Importantly, the substantially constant radii saddle shaped articulationsurfaces 204, 304 are configured and sized to be nestable against oneanother and articulatable against one another, to enable adjacentvertebral bones (against which the upper and lower elements 200, 300 arerespectively disposed in the intervertebral space) to articulate inflexion, extension, and lateral bending. More particularly, as bestshown in FIGS. 1–5, the artificial disc implant 100 of the presentinvention is assembled by disposing the upper 200 and lower 300 elementssuch that the vertebral body contact surfaces 202, 302 are directed awayfrom one another, and the articulation surfaces 204, 304 are nestedagainst one another such that the concave arc 212 accommodates theconvex arc 312 and such that the convex arc 214 is accommodated by theconcave arc 314. Either during or after such assembly of the implant100, the vertebral body contact surface 202 of the upper element 200 isfixed against a lower endplate of a superior vertebral body (not shown),and the vertebral body contact surface 302 of the lower element 300 isfixed against an upper endplate of an inferior vertebral body (notshown). As noted above, the preferable long-term and short-term fixationstructures on the elements 200, 300 are useful for securing the elements200, 300 to these adjacent vertebral bones.

Accordingly, movement of the adjacent vertebral bones relative to oneanother is permitted by the movement of the upper 200 and lower 300elements relative to one another. With regard to the articulationsurfaces 204, 304 being configured and sized to enable the adjacentvertebral bones to articulate in flexion, extension, and lateralbending, it is understood from the described geometry and positioning ofthe upper 200 and lower 300 elements once the implant 100 is assembledand implanted that in flexion and extension, the concave arcs (e.g.,212) of the upper element 200 ride on the convex arcs (e.g., 312) of thelower element 300 about a center of rotation (referenced as R3 on FIG.18) at the center of the circle defined by the convex arc 312. Thiscenter of rotation R3 is below the articulation surface 304. It isfurther understood from the described geometry and positioning of theupper 200 and lower 300 elements that in lateral bending, the concavearcs (e.g., 314) of the lower element 300 ride on the convex arcs (e.g.,214) of the upper element 200 about a center of rotation (referenced asR2 on FIG. 12) at the center of the circle defined by the convex arc214. This center of rotation R2 is above the articulation surface 204.During these articulations, the elements 200, 300 are maintained atsubstantially constant relative distraction positions, i.e, the elements200, 300 do not significantly move (if at all) in directions that aredirected away from one another (for example, do not move in opposingaxial directions from one another (e.g., along the longitudinal axis ofthe spine)). Accordingly, the present invention provides a pair ofarticulation surfaces 204, 304 that have a center of rotation above thesurfaces in one mode of motion (lateral bending), and below the surfacesin another (flexion/extension), consistent in these regards with anatural intervertebral joint in the cervical spine. Preferably, thearticulation surfaces 204, 304 are sized and configured so that therespective ranges of angles through which flexion/extension and lateralbending can be experienced are equal to or greater than the respectivenormal physiologic ranges for such movements in the cervical spine.While the present invention is not limited to any particular dimensions,a preferred embodiment has the following radii of curvature for theconvex arc 312 and the convex arc 214: C=0.280 inches and B=0.340inches. Such preferable radii of curvature provide the preferredembodiment with a flexion/extension range of plus or minus 7.5 degrees(total of 15 degrees), and a lateral bending range of plus or minus 7.5degrees (total of 15 degrees).

While the preferred embodiment is shown with concave arc 212 having alarger constant radius of curvature A than the constant radius ofcurvature C of convex arc 312 (for reasons that are described in detailbelow), and with concave arc 314 having a larger constant radius ofcurvature D than the constant radius of curvature B of convex arc 214(for reasons that are described in detail below), it should beunderstood that the above described functionality can also be achievedusing other relative radii sizes. For example, flexion, extension, andlateral bending are also possible if the constant radius of curvature Aof concave arc 212 is otherwise non-congruent with (e.g., less than) oris congruent with (i.e., equal to) the constant radius of curvature C ofconvex arc 312, and/or if the constant radius of curvature D of concavearc 314 is otherwise non-congruent with (e.g., less than) or iscongruent with (i.e., equal to) the constant radius of curvature B ofconvex arc 214.

As noted above, it is preferable that, in addition to the flexion,extension, and lateral bending motions described above, the adjacentvertebral bones be permitted by the artificial disc implant 100 toaxially rotate relative to one another (e.g., about the longitudinalaxis of the spinal column), through a range of angles without moving inopposite (or otherwise directed away from one another) directions (e.g.,along the longitudinal axis) within that range. Preferably, thearticulation surfaces 204, 304 are accordingly configured and sized topermit such movement. Referring again to FIGS. 1–5, a preferredconfiguration is shown as an example, where the constant radius ofcurvature A of concave arc 212 is larger than the constant radius ofcurvature C of convex arc 312, and the constant radius of curvature D ofconcave arc 314 is larger than the constant radius of curvature B ofconvex arc 214. It is understood from the described geometry andpositioning of the upper 200 and lower 300 elements that, because of thespace, afforded by the differing radii, at the edges of the articulationsurfaces 204, 304, the upper 200 and lower 300 elements are able toaxially rotate relative to one another (e.g., about the longitudinalaxis) through a range of angles without causing the vertebral bodycontact surfaces 202, 302 to move in opposite (or otherwise directedaway from one another) directions (e.g., along the longitudinal axis).Once the axial rotation exceeds that range, the articulation surfaces204, 304 interfere with one another as the concave arcs 212, 314 movetoward positions in which they would be parallel to one another, and thedistance between the vertebral body contact surfaces 202, 302 increaseswith continued axial rotation as the concave arcs 212, 314 ride upagainst their oppositely directed slopes. Thus, the articulationsurfaces 204, 304 are configurable according to the present invention topermit normal physiologic axial rotational motion of the adjacentvertebral bones about the longitudinal axis of the spinal column througha range of angles without abnormal immediate axially opposite (orotherwise directed away from one another) movement, and to permit suchaxially opposite (or otherwise directed away from one another) movementwhen under normal physiologic conditions it should occur, that is,outside that range of angles. While the present invention is not limitedto any particular dimensions, a preferred embodiment has the followingradii of curvature: A=0.329 inches, B=0.340 inches, C=0.280 inches, andD=0.401 inches. Such preferable radii of curvate provide the preferredembodiment with a longitudinal axial rotation range of plus or minus 3degrees (total of 6 degrees) before oppositely directed movement of thearticulating surfaces occurs.

It should be noted that in the preferred embodiment, and in otherpreferable embodiments where the constant radius of curvature A ofconcave arc 212 is larger than the constant radius of curvature C ofconvex arc 312, and the constant radius of curvature D of concave arc314 is larger than the constant radius of curvature B of convex arc 214,the articulation surfaces 204, 304 maintain point-to-point contact overa range of normal physiologic articulating movement between the adjacentvertebral bones. This is illustrated in FIGS. 4, 5, 20, and 21. Moreparticularly, it is understood from the described geometry andpositioning of the upper 200 and lower 300 elements that throughflexion, extension, lateral bending, and axial rotation, thearticulation surfaces 204, 304 are in point-to-point contact with oneanother as they are in FIGS. 4 and 5. FIGS. 20 and 21 are provided toshow the implant 100 in extension and lateral bending, respectively, tofurther illustrate this preferable feature.

It should further be noted that in addition to the radii of curvaturedimensions of the articulation surfaces 204, 304 being relevant to aconfiguration and sizing of the articulation surfaces 204, 304permitting normal physiologic flexion, extension, lateral bending, andaxial rotation movements of the adjacent vertebral bones, the surfacearea dimensions are also relevant, particularly in relation to theselected radii of curvature. More particularly, in order to provide arange of relative angulation that is within the normal physiologic rangeof the cervical spine, not only must the selected radii of curvature besuitable as described above, but also and accordingly the surface areaof the saddle surfaces should be of a dimension that, given the selectedradii of curvature, prevents the edges of the saddle surfaces(particularly the edges of the concave arcs (e.g., 212 and e.g., 314))from hitting any surrounding anatomic structures, or other portions ofthe opposing element (200 or 300), before the limit of the normalphysiologic range of the attempted articulation is reached. As shown,one or both of the inwardly facing surfaces of the upper 200 and lower300 elements can be tapered inwardly before presenting its articulationsurface (204 or 304), to ensure a suitable surface area dimension toprevent such interference. While the present invention is not limited toany particular surface area dimensions, the illustrated preferredembodiment has a surface area of articulation surface 204 equal to 0.146square inches, and a surface area of articulation surface 304 equal to0.153 square inches.

Further preferably, the articulation surfaces 204, 304 are formed ofcobalt-chrome that is polished to provide a smooth bearing surface. Itshould be understood that the articulation surfaces 204, 304, whilepreferably formed of cobalt-chrome, can be additionally or alternativelyformed of other metals, such as, for example, stainless steel and/ortitanium, or of non-metals, such as, for example, polyethylene and/orceramic materials (e.g., alumina and zirconia), or of any other suitablematerial without departing from the scope of the present invention.

It should be noted that while the present invention is illustrated anddescribed as an artificial disc implant for use in the cervical spine,the artificial disc implant of the present invention can be adapted foruse in any other portion of the spine without departing from the scopeof the present invention.

While the particular prostheses for the cervical intervertebral joint ofthe spine as herein shown and disclosed in detail are each fully capableof obtaining the objects and providing the advantages previously stated,it shall be understood that these variations are merely illustrative ofthe presently preferred embodiments of the invention and that nolimitations to the scope of the present invention are intended to beinferred from the details of the construction or design herein shown.

1. An apparatus for replacing at least a portion of an intervertebraldisc in a spinal column, comprising: a first member having a firstvertebral contact surface for engagement with an endplate of a firstvertebral bone in the spinal column and a first articulating surface,the entirety of the first articulating surface being a single saddlesurface that is defined by a concave arc having a substantially constantradius of curvature A about a first axis perpendicular to an axispassing through leading and trailing ends of the first member and aconvex arc having a substantially constant radius of curvature B about afirst axis perpendicular to an axis passing through lateral ends of thefirst member; a second member having a second vertebral contact surfacefor engagement with an endplate of a second vertebral bone in the spinalcolumn, and a second articulating surface in contact with the firstarticulating surface, the entirety of the second articulating surfacebeing a single saddle surface that is defined by a convex arc having asubstantially constant radius of curvature C about a first axisperpendicular to an axis passing through leading and trailing ends ofthe second member and a concave arc having a substantially constantradius of curvature D about a second axis perpendicular to an axispassing through lateral ends of the second member, wherein the constantradius of curvature A is non-congruent with the constant radius ofcurvature C and the constant radius of curvature B is non-congruent withthe constant radius of curvature D; an intervertebral disc space isdefined substantially between the first and second endplates of thefirst and second vertebral bones; and the first and second members areoperable to articulate relative to one another, when disposed in theintervertebral disc space, about at least one of: (i) a first center ofrotation for at least one of flexion and extension that is located abovethe first and second articulating surfaces, and (ii) a second center ofrotation for lateral bending that is located below the first and secondarticulating surfaces.
 2. The apparatus of claim 1, wherein: the firstarticulation surface has a toroidal saddle shape; and the secondarticulation surface has a toroidal saddle shape, wherein the first andsecond articulation surfaces are sized and shaped to engage one anotherwhen the first and second members are disposed in the intervertebraldisc space to enable at least one of flexion, extension, and lateralbending.
 3. The apparatus of claim 1, wherein the first and secondarticulation surfaces are sized and shaped to engage one another whenthe first and second members are disposed in the intervertebral discspace to enable the first and second vertebral bones to at least axiallyrotate relative to one another through a range of angles.
 4. Theapparatus of claim 3, wherein the radius A of the concave arc is greaterthan the radius C of the convex arc in order to permit axial rotation ofthe first and second vertebral bones relative to one another.
 5. Theapparatus of claim 3, wherein the radius D of the concave arc is greaterthan the radius B of the convex arc in order to permit axial rotation ofthe first and second vertebral bones relative to one another.
 6. Theapparatus of claim 3, wherein the first and second articulation surfacesare sized and shaped to achieve substantial point-to-point contactrelative to one another when the spinal column is in at least somepositions of flexion, extension, lateral bending, and/or axial rotation.7. The apparatus of claim 3, wherein the first and second articulationsurfaces are sized and shaped to engage one another when the first andsecond members are disposed in the intervertebral disc space to enablethe first and second vertebral bones to axially rotate relative to oneanother through the range of angles without substantially displacing thefirst and second vertebral bones away from one another.
 8. The apparatusof claim 7, wherein the range of angles is about plus/minus threedegrees from a resting position.
 9. The apparatus of claim 7, whereinthe first and second articulation surfaces are sized and shaped suchthat the first and second vertebral bones are displaced away from oneanother at axial rotations outside the range of angles.
 10. Theapparatus of claim 1, wherein at least one of: (i) the first and secondaxes perpendicular to the axis passing through the leading and trailingends are substantially coaxial; and (ii) the first and second axesperpendicular to the axis passing through the lateral ends aresubstantially coaxial.
 11. The apparatus of claim 1, wherein at leastone of: (i) the first and second axes perpendicular to the axes passingthrough the leading and trailing ends lie in a plane that issubstantially perpendicular to the anterior-posterior plane; and (ii)the first and second axes perpendicular to the axes passing through thelateral ends lie in a plane that is substantially perpendicular to thelateral plane.
 12. An apparatus for replacing at least a portion of anintervertebral disc in a spinal column, comprising: a first means havinga first surface for engagement with an endplate of a first vertebralbone in the spinal column; a second means having a second surface forengagement with an endplate of a second vertebral bone in the spinalcolumn, wherein the first and second means have opposing first andsecond articulating surfaces that are in contact with one another, theentirety of the first articulating surface being a single saddle surfacethat is defined by a concave arc having a substantially constant radiusof curvature A about a first axis perpendicular to an axis passingthrough leading and trailing ends of the first means and a convex archaving a substantially constant radius of curvature B about a first axisperpendicular to an axis passing through lateral ends of the firstmeans, the entirety of the second articulating surface being a singlesaddle surface that is defined by a convex arc having a substantiallyconstant radius of curvature C about a first axis perpendicular to anaxis passing through leading and trailing ends of the second means andconcave arc having a substantially constant radius of curvature D abouta second axis perpendicular to an axis passing through lateral ends ofthe second means, the constant radius of curvature A being non-congruentwith the constant radius of curvature C and the constant radius ofcurvature B being non-congruent with the constant radius of curvature D;an intervertebral disc space is defined substantially between the firstand second endplates of the first and second vertebral bones, and thefirst and second means are operable to articulate relative to oneanother, when the first and second means are disposed in theintervertebral disc space, about at least one of: (i) a first center ofrotation for at least one of flexion and extension that is located abovethe first and second articulating surfaces, and (ii) a second center ofrotation for lateral bending that is located below the first and secondarticulating surfaces.
 13. An apparatus for replacing at least a portionof an intervertebral disc in a spinal column, comprising: a first memberhaving a first vertebral contact surface for engagement with an endplateof a first vertebral bone in the spinal column, and having a firstarticulation means, the entirety of the first articulating means being asingle saddle surface that is defined by a concave arc having asubstantially constant radius of curvature A about a first axisperpendicular to an axis passing through leading and trailing ends ofthe first member and a convex arc having a substantially constant radiusof curvature B about a first axis perpendicular to an axis passingthrough lateral ends of the first member; and a second member having asecond vertebral contact surface for engagement with an endplate of asecond vertebral bone in the spinal column, and having a secondarticulation means the entirety of the second articulating means being asingle saddle surface that is defined by a convex arc having asubstantially constant radius of curvature C about a first axisperpendicular to an axis passing through leading and trailing ends ofthe second member and a concave arc having a substantially constantradius of curvature D about a second axis perpendicular to an axispassing through lateral ends of the second member, wherein the constantradius of curvature A is non-congruent with the constant radius ofcurvature C and the constant radius of curvature B is non-congruent withthe constant radius of curvature D; an intervertebral disc space isdefined substantially between the first and second endplates of thefirst and second vertebral bones, and the first and second articulationmeans are operable to articulate relative to one another, when the firstand second members are disposed in the intervertebral disc space, aboutat least one of: (i) a first center of rotation for at least one offlexion and extension that is located above the first and secondarticulation means, and (ii) a second center of rotation for lateralbending that is located space below the first and second articulationmeans.